Eukaryotic initiation factor 2, eIF-2, plays a regulatory role in the initiation of protein synthesis in animal cells. eIF-2 activity is modulated by phosphorylation, guanine nucleotides and the redox state of its sulfhydryl groups and has been used as a model protein in which to study these three important regulatory mechanisms of animal cell function. eIF-2 is an allosteric protein whose activity is modulated by sequential changes in its conformation to fulfill its function at different stages of its activity cycle. Phosphorylation of the alpha-subunit does not directly reduce its Met-tRNAf binding ability, but prevents one of the sequential conformational transitions such that the Beta-subunit of eIF-2 is unable to interact with other components of the eIF-2 activity cycle.